As a power generation system for a combined cycle electric power generation plant, a so-called exhaust heat recovery system for leading exhaust gas after work in a gas turbine to a heat recovery steam generator (H.R.S.G.), generating steam using the exhaust heat, and driving a steam turbine using the steam is generally used.
In a combined cycle electric power generation plant of such an exhaust heat recovery system, efforts for refinement and improvement are always performed for improvement of efficiency and stability of operation.
For example, in a gas turbine, in order to realize improvement of efficiency by raising of combustion temperature, the temperature of a fuel gas is raised. Specifically, a fuel gas heater for heating the fuel gas of the gas turbine is provided. The fuel gas heater heats the fuel gas with intermediate-pressure feed water supplied to the heat recovery steam generator. The heated fuel gas is supplied to a combustor of the gas turbine and consumed (see Patent Document 1 described below).
As an example of heat balance in the fuel gas heater, as shown in FIG. 7, the temperature of a fuel gas supplied to a fuel gas heater 172 is 6.8° C. and the temperature of intermediate-pressure feed water supplied from an intermediate-pressure economizer 139 to the fuel gas heater 172 is 255° C. The temperature of a fuel gas discharged from the fuel gas heater 172 after heat exchange and supplied to a combustor 112 of a gas turbine 100 is 210° C. and the temperature of intermediate-pressure feed water also discharged from the fuel gas heater 172 after heat exchange and supplied to a pre-heater 137 is 65° C.
In order to prevent overheating of the turbine and realize stable operation of the gas turbine, moving blades and stationary blades of the turbine are cooled. For this cooling, compressed air extracted from a compressor of the gas turbine is used. Since the compressed air is heated by compression, a cooling air cooler for cooling this compressed air is provided. The cooling air cooler cools, with high-pressure feed water supplied to the heat recovery steam generator, the compressed air extracted from the compressor of the gas turbine. The cooled air is supplied to the moving blades and the stationary blades of the turbine and cools the moving blades and the stationary blades (see, for example, Patent Document 2 described below).
As an example of heat balance in the cooling air cooler, as shown in FIG. 7, the temperature of high-pressure feed water supplied from the pre-heater 137 to the conventional cooling air cooler 171 is 172° C. and the temperature of compressed air supplied from a compressor 111 of the gas turbine 100 to the cooling air cooler 171 is 456° C. The temperature of high-pressure feed water discharged from the cooling air cooler 171 after heat exchange and supplied to a high-pressure drum 134 is 326° C. and the temperature of compressed air also discharged from the cooling air cooler 171 after heat exchange and supplied for cooling of the turbine 113 is 200° C.    Patent Document 1: Japanese Patent Laid-Open No. 2003-343283    Patent Document 2: Japanese Patent Laid-Open No. H10-169414